This undated photo provided by the Fresno Police Department shows Lawrence Jones. Jones, a parolee who worked at a California chicken processing plant, pulled a gun and opened fire at the business on Tuesday, Nov. 6, 2012 killing at least two people and wounding two others, before taking his own life, authorities said. (AP Photo/Fresno Police Department)

Lawrence Jones, a parolee who worked at a California chicken processing plant, pulled a gun and opened fire at his co-workers

AP Photo/Fresno Police Department

(CBS/AP) FRESNO, Calif. - Authorities said it remains unclear why Lawrence Jones targeted a handful of his co-workers at a Fresno chicken processing plant on Tuesday and killed two of them execution-style .

"He had opportunity to shoot other co-workers that were in the business at this time, but he chose not to," Fresno Police Chief Jerry Dyer said. "He walked around them in order to get very close to the intended targets, place the gun very close and fire a round."

Authorities said Jones fatally shot two people and wounded two others before he ran out of bullets when he fired at a fifth person. Police said he later reloaded and shot himself in the head. He was pronounced dead at a hospital a few hours later.

The 42-year-old ex-convict moved methodically from victim to victim, placing his handgun against their head or neck and then pulling the trigger, authorities said. Clattering machinery at the plant provided all the cover he needed for the shooting, drowning out the gunshots as the unsuspecting victims had on ear protectors as they worked just feet apart at Valley Protein.

Jones' motive was unknown, but it was clear he had targeted his victims, Dyer said. About 30 employees were working at the plant during the shooting.

Police said that some workers told them Jones did not appear to be himself when he arrived for his shift.

"We are still trying to follow up on some rumors regarding a dispute between Jones and one of the other co-workers, but we have not been able to verify it at this point," Dyer said.

Authorities said Jones had an extensive criminal history. In 1994, he was sentenced to 13 years in prison for robbery, and then released on parole in 2001, according to the state Department of Corrections and Rehabilitation. He also served time for robbery, vehicle theft and other convictions, and was paroled on June 1, 2011, department spokesman Luis Patino said. He was discharged from parole on May 1, 2012.

Dyer said Jones pulled out his handgun about four hours into his shift and walked up to 32-year-old Salvador Diaz in the grinding room and pulled the trigger. Diaz was pronounced dead at the scene.

Jones then went next door to the deboning room and shot 34-year-old Manuel Verdin, who was pronounced dead at a hospital. Jones also wounded 28-year-old Arnulfo Conrriguez and 32-year-old Fatima Lopez. Hospital spokeswoman Mary Lisa Russell said Conrriguez was in serious condition at Fresno's Community Regional Medical Center while Lopez was treated and released.

Plant worker Estevan Catano, 21, escaped injury when the gun Jones placed against his head did not discharge because it was out of ammunition, police said. The gun only held four rounds.

Investigators secured Jones' Fresno apartment, where they found several firearms. Police said the handgun used by Jones - a .357 Derringer - is an expensive and rare weapon and they are trying to determine whether it was stolen. The serial numbers on the gun were filed off, Dyer said.